megger testing

[ranger12]

how does one interpret the results of megohmeter tests officially? i usually connect the black lead to a known good ground reference source to begin with.Then i open the wire or the circuit to be tested.then i crank up the megger.it is my understanding that this test measures the quality of the insulation by introducing a high voltage/low current thru the conductor being tested.usually in my experience it is pass or fail.What i don't get though is how does the megger tell me that a wire is bad when one side of the wire is obviously open?

[friebel]

ranger12, There is some information that you have left out in order to get a good answer for megger testing.
1.What voltage is the system that you are testing?
2.It sounds to me that you want to test the insulation of your wiring.
3.I am going to assume that you are testing wiring for a 480vac system.
4.I would set the megger to check a 480vac system with 1000vdc from the megger.
5.Connect one lead of the megger to the conductor, and the other lead to ground, and see what reading you get with the megger.
6.The One-Megohm Rule, I am going to quote out of a book featuring Megger Insulation Testing.It is called a "Stitch in Time".
Quote: Insulation resistance should be approximately one megohm for each 1000 volts of operating voltage with a minimum value of one megohm. For example, a motor rated at 2400 volts should have a minimum insulation resistance of 2.4 megohms.End of Quote.
ranger12, there is a lot to learn about insulation testing that I cannot cover with this short message.
I am a new member, and I do not know the guidelines for giving out telephone numbers, vendors etc. I would be happy to send you a copy of "A Stitch in Time", The Complete Guide to Electrical Testing, featuring MEGGER Insulation Testers. Or you could contact the AVO company, and they would send you a copy.
Regards, bob

[bonding jumper]

Check this post TESTING WIRE FROM FIRE

friebel you can Private Message (PM) your phone # to ranger12 by clicking on the PM icon in the ranger12 post.

[ranger12]

Thank you for responding Friebel.In this instance i was at amobile home that had a stick framed addition added to it.State Labor and Industries wer'e asked to inspect the addition as no permit had been pulled when it was constructed.They required megger testing of the wiring.It was a 120/240 voltage system.I assumed as far as this correction was concerned that they wanted to assure that the wiring insulation was allright as they never had a chance to inspect it in the first place.I also addressed all the other obvious things like polarity and broken plugs and grounded switches.but really i am trying to learn and understand how the megger works.I did a search on the avo company and came up emptyhanded.If you could give me a site to go to i would appreciate it.I would be happy to buy a book from them if it is worthwhile .Thanks

[friebel]

Good morning, ranger12,In reponse to your message that you could not find the AVO company on the internet.
Type www.avointl.com I typed it in and boy, I did not realize the information that you can receive. I am always amazed at the info. on the internet.
ranger12 I have the megger books, that AVO has sent to me for my classes. I would be glad to send one to you. You could send me your address on a private message. But, it would be a good idea for you to contact AVO and communicate with them. I live in south Jersey, and I deal with AVO in Valley Forge, Pennsylvannia.
There is so much to learn about Megger Insulation Testing, and how to properly use the Megger.

[pierre]

Friebel
That is a great link, Thanks

Pierre

[brian john]

This has been covered BUT the one megaohm rule IMO is not an acceptable rule/standard. Way too low, Check NETA for test spec.s

http://www.netaworld.org/Category.cfm?CategoryID=3

[don_resqcapt19]

Brian,
Why is that too low? On a 480 volt system, a 1,000,000 ohm resistance between the circuit and grounding conductors would result in leakage current of 0.00048 amps or about 1/10 of the current that will casue a GFCI to trip.
Don

[friebel]

To: brian john, You are correct, the one megohm is too low. I should have explained myself in more detail. If I had a motor that was critical to the operations, and had a one-megohm reading, I would allow the operations to continue to run this motor. But I would start preparations to replace this motor as soon as possible.
It has been my experience that most electricians are not familiar with "what is the minimum reading in megohms" that you can continue to run a motor.
The AVO manual states: one megohm per thousand volts, with a minimum reading of one megohm.
Example: A 2400 volt motor would need a minimum reading of 2.4 megohms.
But a 460 volt motor minimum reading could be no lower than one-megohm.
I have worked on an industrial plant that had about 10,000 motors, from 2300 volts to 575 volts, 460 volts. So in my 45 years on this plant I have done a great deal of testing of motors.
But again brian john, you are correct in your assessment, the one-megohm is to low.